Patent Application
20190157686
The invention relates to a method for producing a supported catalyst material for a catalytic fuel-cell electrode. The invention further relates to a catalyst material that can be produced by this method, an electrode structure for a fuel-cell with such a material, and a fuel-cell having such an electrode structure.
Fuel-cells use the chemical conversion of a fuel with oxygen into water in order to generate electrical energy. For this purpose, fuel-cells contain the so-called membrane electrode assembly (MEA) as a core component, which is an arrangement of an ion-conducting (usually proton-conducting) membrane and of a catalytic electrode (anode and cathode), respectively arranged on both sides of the membrane. The latter generally comprise supported precious metals—in particular, platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane electrode assembly, on the sides of the electrodes facing away from the membrane. Generally, the fuel-cell is formed by a plurality of MEA's arranged in the stack, the electrical power outputs of which add up. Bipolar plates (also called flow field plates or separator plates), which ensure a supply of the individual cells with the operating media, i.e., the reactants, and which are usually also used for cooling, are generally arranged between the individual membrane electrode assemblies. In addition, the bipolar plates also ensure an electrically-conductive contact to the membrane electrode assemblies.
During operation of the fuel-cell, the fuel (anode operating medium)—particularly, hydrogen H2 or a gas mixture containing hydrogen—is supplied to the anode via an open flow field of the bipolar plate on the anode side, where electrochemical oxidation of H2 to protons H+ with loss of electrons takes place (H2→2H++2 e−). Protons are transported (water-bound or water-free) from the anode chamber into the cathode chamber across the electrolyte or membrane that separates and electrically insulates the reaction chambers, gas-tightly, from each other. The electrons provided at the anode are guided to the cathode via an electrical line. The cathode receives, as a cathode operating medium, oxygen or a gas mixture containing oxygen (such as air) via an open flow field of the bipolar plate on the cathode side, so that a reduction of O2 to O2− with gain of electrons takes place (½O2+2 e−→O2−). At the same time, the oxygen anions react in the cathode chamber with the protons transported across the membrane to form water (O2−+2H+→H2O).
Platinum or platinum alloys are used as catalytic materials for catalyzing the aforementioned fuel-cell reactions. Since the reactions involve an electrochemical surface process, the largest possible catalytic surface (ECSA) is sought. For this purpose, particles of the catalytic material in the size range of a few nanometers are applied to a carbon carrier with a large surface area. In the course of fuel-cell operation, however, some of the electrical power is lost due to electrode degradation. The main reason for this is the loss of ECSA and activity due to (unfavorable) operating conditions. The underlying mechanisms include, among other things, severing of the platinum from the carbon carrier (platinum corrosion), whereby the particles lose their electrical contact and no longer contribute to the effective catalysis. Also involved is the agglomeration (coalescence) of the particles, whereby the catalytic surface decreases. Further degradation mechanisms include corrosion of the alloying elements, cobalt or nickel, but also corrosion of platinum itself, growth of the platinum nanoparticles by Ostwald ripening, growth of the platinum nanoparticles by migration, and sintering on the carbon surface.
In order to counteract the loss of catalytic activity, and thus to ensure the performance requirements over the operating time of the fuel-cell, an excess amount of the precious metal is usually used in the manufacture of the electrodes. However, this measure is quite cost-intensive.
The use of stabilized carbon carriers is also known. While this corrosion (disintegration) of the carbon is improved, the adhesion of the catalyst particles is not.
Furthermore, nanostructured thin-film catalysts are known in which the amount of platinum can be reduced by increased service life. However, the nanostructure causes problems in the removal of the reaction water.
In addition, the activity of the catalyst is increased by alloying other elements (especially, cobalt and nickel) in order to ensure a higher fuel throughput and thus a high electrical power. However, the problem of lack of adhesion is not eliminated by the addition of alloying elements. Owing to the non-precious character of these elements compared to platinum, such catalysts are even significantly more susceptible to corrosion.
There is also research work with new catalyst carrier concepts in which adhesion promoter layers on an oxidic basis are to improve the adhesion of the catalyst material. However, the oxides used are usually poor conductors, so that power losses occur due to contact resistances between catalyst and carrier.
DE 698 24 875 T2 describes the production of non-conductive, nanostructured carrier structures from an organic pigment on a carrier film. These carrier structures are coated by means of physical or chemical vapor deposition (PVD, CVD) in order to produce nanostructured catalyst elements, which are then transferred directly to the polymer electrolyte membrane of the fuel-cell by a pressing process. The catalyst elements can thereby have different compositions on their surface and in their internal volume by sequential deposition of different materials.
In addition, the admixture of metal carbides to the catalytic material or the direct application of metal carbides to the carbon carrier is known (for example, EP 1842589 A1). In this case, there is, at most, a physical adhesion of the carbide to the carrier material.
US 2006/0183633 A1 describes a catalyst structure for the anode of a direct methanol fuel-cell (DMFC). This comprises a carrier material of Al, Ti, TiN, W, Mo, or Hf on which local elevations (nanodots) are deposited from a metal carbide, such as WC, MoC or TaC, and on these catalytic particles by means of physical or chemical vapor deposition. Both the bumps and the catalytic particles consist of a metal carbide, such as WC, MoC, or TaC, and may optionally be provided with a carbon nanohorn (CNH) coating.
The catalytic particles are generally present on an electrically-conductive carrier material of large specific surface area, which is often a particulate carbon-based material—for example, carbon nanotubes (CNT) or the like. The deposition of the catalytic particles on the carrier material is usually carried out with wet-chemical methods, e.g., via sol-gel process, using metal-organic precursor compounds of the catalytic metal (for example, U.S. Pat. No. 8,283,275 B2). In addition, the deposition of catalytic precious metal particles onto the carbon carrier from the gas phase is also known (for example, U.S. Pat. No. 7,303,834 B2). Subsequently, the catalyst thus supported is mixed with an ionomer and applied, in the form of a paste or suspension as a coating to a carbon paper, directly onto the polymer electrolyte membrane or to the gas diffusion layer, and dried.
The aim of the invention is to provide a method for producing a supported catalyst material for a catalytic fuel-cell electrode which leads to a material that at least partially solves the problems of the prior art. In particular, a catalyst material is to be produced which has an improved adhesion of the catalytic material to the carrier material and, thus, an increased stability and longer service life.
This aim is achieved by a production method, a supported catalyst material which can be produced by the method, an electrode structure with such a material, and a fuel-cell having such an electrode structure with the features of the independent claims. Additional preferred embodiments of the invention arise from the other features stated in the sub-claims and the following description.
The method according to the invention for producing a supported catalyst material for a fuel-cell electrode comprises the following steps (in particular, in the order indicated):
The invention is thus characterized in that a chemical bond between the carbide-forming substance and the carrier material is produced by creation of a carbide or a carbide-like bond between the substance and the carbon. This provides an improvement in the bonding of the catalyst material to the surface. In known methods in which the catalytic material is separated out from the gas phase by a wet-chemical or purely physical deposition process, there is only one physical connection by adsorption, which is naturally weaker than a chemical (covalent) bond. Because of the stable joining, the diffusion of the catalytic structures on the carbon carrier surface, and therefore its coalescence (sintering), is prevented. Carbides are also distinguished by a high mechanical and chemical stability, and also by a high electrical conductivity. Thus, power losses due to contact resistances between catalyst and carrier can be minimized.
It goes without saying that the carbide-containing layer does not necessarily have to form its own carbide phase, e.g., in the form of nanocrystallites grown on the surface that have a crystallographic carbide structure. Rather, it is sufficient for carbide bonds or carbide-like bonds to form at the immediate interface between carbon and the carbide-containing layer, which means that there is a covalent bond between the carbide-forming element and the carbon, so that chemical bonding conditions are present which locally correspond to those in a carbide crystal, but do not have the periodicity and long-range order corresponding to a crystal. In addition, a phase of the pure, unreacted carbide-forming substance, which is also part of the carbide-containing layer, can be applied to this boundary layer.
Furthermore, at the boundary layer between the carbide-containing layer and the catalytic precious metal layer, a stable alloy is formed between the carbide-forming substance (hereinafter also referred to as carbide former) and the precious metal or its alloy. A stable attachment of the catalyst to the carbon carrier over all interfaces is thus achieved.
The deposition of the carbide-forming substance and deposition of the catalytically-active precious metal or its alloy is from the gas phase, and thus not by a wet-chemical process from a solvent. By the deposition from the gas phase, it is possible to achieve a targeted structuring at an atomic level that cannot be provided via classical, wet-chemical synthesis routes. Moreover, the vapor deposition allows a significant reduction in the amounts used—in particular, of the catalytic precious metal or its alloy. In this way, only precious metal is used on the surface on which the catalytic reaction takes place, while a less costly material is present in the interior.
The depositions may be done with any gas phase deposition process. Suitable methods include physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), and others.
The main task of the carbide-containing layer is to ensure stable adhesion of the precious metal or its alloy to the carrier material and, at the same time, to form the catalytic structure in its interior by a comparatively inexpensive material. In order to obtain a good bond to the carbon carrier, a suitable crystal lattice of the carbide or the substance forming the carbide is advantageous, i.e., a similar crystal structure and a lattice constant similar to that of the carbon material. To further provide stable attachment of the catalytic surface layer of precious metal, it is also desirable to have a high surface energy and a lattice structure that is compatible with the catalytic precious metal or its alloy, wherein it is preferred here that the lattice constant of the carbide or the carbide-forming substance correspond at most to that of the catalytic surface layer—in particular, that of platinum. By selecting a lattice of the carbide with a slightly lesser lattice constant, a contraction of the precious metal lattice and, by the resulting distorting effects of the crystal lattice, as well as by quantum mechanical interactions between the lattices, an increased activity, as well as a particularly dense ball packing of the precious metal, are achieved. Overall, the adhesion is determined by the surface energy, a good concordance between the crystal lattices with respect to symmetry and lattice parameters, as well as the bonding states at the boundary surface (displacement of the d-band center). In order to allow the uptake or release of electrons during the catalytic fuel-cell reactions taking place at the catalytically-active precious metal, the volume material of the core layer should also have a good electrical conductivity.
Suitable carbide-forming substances meeting these criteria include titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), boron (B), vanadium (V), aluminum (Al), scandium (Sc), yttrium (Y), silicon (Si), chromium (Cr), and nickel (Ni), or a mixture of two or more of these elements. Of these, titanium, which reacts with carbon to form titanium carbide TiC, is particularly preferred.
For the vapor deposition of the carbide-forming substance, either the pure element itself or a chemical precursor compound therefrom is used, transferred into the gas phase, and deposited on the surface of the carbon-based carrier material. This leads to the corresponding carbide or the carbide-like bond being formed spontaneously by chemical reaction with carbon.
The layer thickness of the carbide-containing layer is preferably selected to be as thin as possible in the region of fewer atomic layers relative to the carbide-forming substances. In an advantageous embodiment, the layer thickness is in the range of 1 atomic layer to 50 atomic layers on average—in particular, an average of 1 atomic layer to 20 atomic layers and, preferably, an average of 1 to 10 atomic layers. These layer thicknesses are sufficient to bring about the desired carbide formation.
Likewise, the layer thickness of the surface layer of the catalytically-active precious metal or its alloy is preferably selected to be as thin as possible, in order to make the expensive material on the surface as completely accessible as possible. In particular, the surface layer has an average layer thickness of 1 to 6 atomic layers—preferably, of 1 to 4 atomic layers and, particularly preferably, of 1 to 2 atomic layers.
The setting of the layer thicknesses of the individual layers in the region of fewer atomic layers allows the targeted use of interface effects for fuel-cell catalysis. The layer thicknesses can be set in a simple manner by suitable selection of the duration of the vapor deposition processes.
Particularly suitable as precious metal for the catalytic surface layer are metals of the platinum group comprising ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), or an alloy of these metals. In particular, the surface layer comprises platinum or a platinum alloy—preferably, essentially pure platinum.
In a preferred embodiment of the invention, the deposition of the carbide-forming substance and the deposition of the catalytically-active precious metal or its alloy take place in a time-overlapping manner, wherein a gradual enrichment of the catalytically-active precious metal or its alloy, or a gradual depletion of the substance forming the carbide in the deposited layer, is produced from inside to outside. This can be done by the relative proportions of the carbide former and the precious metal or its alloy in the gas phase being changed continuously or stepwise during the deposition process. For example, the proportion of the carbide former may be varied from 100% to 0% and that of the precious metal or its alloy from 0% to 100% during the deposition process. Such an adjustment of a gradient is easy to provide with the aforementioned vapor deposition processes. The product is a continuous layer structure, in which the content of the carbide former decreases from inside to outside, and the content of the precious metal or its alloy increases correspondingly. Advantageously, the outermost layer, or at least the outermost atomic layer, consists up to 100% of the catalytic precious metal or its alloy.
According to a further embodiment of the method, a diffusion barrier layer is deposited on the carbide-containing layer after the deposition of the carbide-forming substance, and before the catalytically-active precious metal or its alloy is deposited. The diffusion barrier layer prevents the diffusion of the electrochemically non-precious material of the carbide former forming the core onto the surface, where the atoms of the carbide former are dissolved out through the corrosive environment in the fuel-cell, and the catalyst is thus destabilized. The diffusion barrier layer is preferably covered as completely as possible by the catalytic precious metal or its alloy in the one further vapor deposition step. Suitable materials for the diffusion barrier include, for example, gold (Au), palladium (Pd), ruthenium (Ru), tungsten (W), osmium (Os), rhodium (Rh), and iridium (Ir), or a mixture or alloy thereof. Gold is particularly preferred.
In a further advantageous embodiment, defect sites and/or functionalizations are produced on the surface of the carbon-based carrier material before the deposition of the carbide-forming substance. Defect sites, in the present case, are understood as defects in the crystal lattice structure of the carbon, i.e., deviations from the remaining lattice structure. The defect sites thus comprise imperfections, i.e., unoccupied lattice sites, but also the presence of foreign atoms (not C) on the carbon lattice sites, which, due to their atomic radius deviating from C, lead to lattice distortion, or of foreign atoms on interstitials, in the sense of an intercalation, which can likewise lead to lattice distortion. Suitable foreign atoms include, for example, nitrogen (N), boron (B), oxygen (0), silicon (Si), and others. The term, functionalizations, means chemical groups that are present covalently bonded to the carbon of the carrier. Such functionalizations include, for example, hydrogen groups (—H), hydroxyl groups (—OH), carboxyl groups (—COOH), and others. In this way, the bonds—in particular, double bonds of the carbon surface—are locally broken, and a functionalization is produced that is available for the reaction with the carbide formers. Both defect sites and functionalizations on the surface of the carbon-based carrier material serve as condensation nuclei for the deposition of the carbide-forming substance, and at the same time promote chemical carbide formation due to their increased reactivity. The local formation thus produced of a carbide-like connection between carbon and carbide formers functions as a core for the further growth of the layer or the catalytic structure on the carbon carrier. The deposition of additional atoms of carbide is preferably carried out at this site and results in the growth of a particle. However, in principle, carbon carriers that have not been pre-treated can also be used. Also, in this case, there are local carbide formations and an increase in the structures around these cores.
Imperfections can be produced by irradiating the carrier material with high-energy electromagnetic radiation or with particle radiation, or by treatment with a gas plasma. Alternatively, a carrier material can be used which already has defect sites. Foreign atoms or functionalizations can also be introduced or generated by treatment with a gas plasma—especially, hydrogen (H2), oxygen (O2), nitrogen (N2), or others—or by treatment with a chemical agent or an acid. The carbon-based carrier material is predominantly designed to provide a large, specific surface area for the applied catalyst and also to establish the electrical connection between the catalytic centers of the material and the external circuit of the fuel-cell. Preferably, the carbon-based carrier material has a porous, particulate, i.e., loose, fill structure. This includes, in particular, spheroidal forms or fibers. Suitable materials include, in particular, carbon nanostructures, e.g., carbon nanotubes, carbon nanorods, carbon nanofibers, carbon nanobands, carbon hollow spheres; as well as graphite, volcano, graphitized carbon, graphene, ketjen black, acetylene black, furnace black, carbon black, activated carbon, and meso phase carbon.
The structure of carbide-containing layer and catalytic layer, and, in some cases, further layers in the form of discrete catalytic structures is preferably present on the carbon-based carrier material. Within the scope of the present invention, the term, “catalytic structures,” is understood to mean structures which are formed (grown, deposited) on the carbon-based carrier material and are arranged discretely, i.e., separately, on the carrier material. Thus, there may be gaps between adjacent catalytic structures in which the carrier material is exposed. The catalytic structures can have an arbitrary shape, e.g., approximately the shape of a ball segment—in particular, a hemisphere. However, it is understood that the structures generally do not have the ideal shape of a spherical surface and are determined, in particular, by the crystal structure of the materials used. Irrespective of their geometric form, the catalytic structures have a type of core-shell structure, more precisely a “cut-open” core-shell structure, in which the “cutting surface” is arranged on the carrier material and in contact therewith.
Another aspect of the present invention relates to a supported catalyst material for a fuel-cell electrode which is producible, in particular, using the method according to the invention, and an electrically-conductive, carbon-based carrier material, a carbide-containing layer deposited on the carrier material, and a catalytic layer of a catalytically-active precious metal or an alloy of such deposited on the surface of the carbide-containing layer.
The material is distinguished by an excellent adhesion of the catalytic layer, and thus a low tendency to corrosion and sintering. Furthermore, a comparatively small amount of the precious metal or its alloy is required, because the interior of the catalytic structures is essentially filled by the less expensive carbide former.
A further aspect of the present invention relates to an electrode structure for a fuel-cell comprising a flat carrier and a catalytic coating, arranged on at least one of the two flat sides of the carrier which comprises the supported catalyst material according to the present invention. Thereby, the flat support is, for example, a polymer electrolyte membrane for a fuel-cell. In this case, it is also referred to as a catalytically-coated membrane (CCM). Alternatively, the flat carrier can be a gas-permeable, electrically-conductive substrate, |e.g., a gas diffusion layer (GDL), or a further carrier layer of the fuel-cell—for example, carbon paper or the like. In the case of a catalytically-coated gas diffusion layer, it is also referred to as a gas diffusion electrode.
The electrode structure can be produced by laminating the catalyst material directly onto the flat support. For this purpose, a suspension or paste is prepared comprising the catalyst material, a solvent, and, if desired, further additives, such as binders or the like, and applied by any method to the flat support and dried.
A further aspect of the present invention relates to a fuel-cell with a polymer electrolyte membrane and in each case a layer arranged on flat sides thereof, at least one of which comprises the supported catalyst material of the invention.
The various embodiments of the invention mentioned in this application may be combined advantageously with one another unless stated otherwise in individual cases.
The invention is explained below in exemplary embodiments, with reference to the respective drawings. The following are shown:
The described supported catalyst material according to
The catalyst material according to
The catalyst materials 20 according to
The catalyst material 20 shown in
The catalyst material 20 shown in
The catalyst materials 20 according to
The material 20 according to
Another version of the catalyst material 20 according to the invention is shown in
In order to manufacture an electrode for a fuel-cell, first, a composition (slurry, paste, or the like) is produced from the catalytic material 20 according to the invention and contains a solvent in addition to the catalytic material 20, and may contain further additives—in particular, a polymeric binder. This composition is then applied to a flat support as a coating, for which any coating process, e.g., coating, spraying, scraping, printing, or the like, can be used. The flat carrier is, in particular, a polymer electrolyte membrane of the fuel-cell, which is preferably coated on both sides with the catalytic material. Alternatively, the catalytic coating can also be applied to a gas diffusion layer or to another gas-permeable, electrically-conductive substrate, such as carbon paper.
During operation of the fuel-cell 10, the hydrogen is supplied via the reactant channels 16 of the anode plate 15a, distributed via the gas diffusion layer 13 on the anode side, and fed to the catalytic anode 12a. Here, a catalytic dissociation and oxidation of hydrogen H2 to protons H+ takes place, with release of electrons, which are removed via the circuit 18. On the other hand, via the cathode plate 15k, the oxygen is conducted to the catalytic cathode 12k via the cathode-side gas diffusion layer 13. At the same time, the proteins H+ formed on the anode side diffuse across the polymer electrolyte membrane 11 in the direction of the cathode 12k. In this case, the supplied atmospheric oxygen reacts to the catalytic precious metal, taking up the electrons supplied via the external circuit 18 with the protons to form water, which is discharged from the fuel-cell 10 with the reaction gas. The electrical load 19 can be supplied by the electrical current flow thus generated.
The catalyst material 20 according to the present invention may be used for the anode 12a and/or the cathode 12k of fuel-cells. The fuel-cell 10 equipped with the catalytic material 20 according to the invention is characterized in that the catalytic electrodes 12a, 12k have a low corrosion tendency, and thus high long-term stability. At the same time, comparatively little catalytic precious metal is required, since the main volume of the catalytic material of the electrodes is formed by a comparatively inexpensive material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2016 111 981.4 | Jun 2016 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/065642 | 6/26/2017 | WO | 00 |
